Roof Truss

Triangles do not flex out of shape under load the way a rectangle does, and that simple geometry is the whole idea behind a roof truss: a rigid frame, usually made of 2x4 lumber joined into a series of triangles, that holds up the roof. A web of triangles can carry the weight of the roof, the sheathing, the tile, and any snow or wind load, then push that weight outward to the home's exterior walls.

Most trusses are designed by an engineer, built in a factory to exact dimensions, then delivered and lifted onto the walls by crane. Because a truss spans from one outside wall to the other, it usually removes the need for interior load-bearing walls. That is why truss-framed homes can have large open great rooms with no posts in the middle. The 2018 International Residential Code, Chapter 8 and Section R802.10, governs how trusses are designed, braced, and connected.

A truss is different from a rafter and a header. A rafter is a single sloped board, cut on site, that runs from the ridge to the wall (called stick framing). A truss is a complete engineered assembly. A header carries load over an opening like a door or window. A truss carries the whole roof.

For example, a custom home in Rio Verde with a 40-foot-wide great room can be roofed with engineered trusses that span the full width, leaving the interior open with no center wall. In Arizona, truss design accounts for the local wind and, in higher-elevation builds near Cave Creek and the Mogollon Rim corridor, snow load. Cutting or drilling a truss in the field is never allowed without an engineer's sign-off, because it is a calculated assembly and altering one member can compromise the whole frame.